Don't Freak Out, But Only 499 Stocks Will Be Trading In The S&P 500 Until Friday

Birinyi Associates pointed us to an announcement from Standard & Poor's published yesterday about changes to the index. Because Google is acquiring Motorola Mobility Holdings, a new company—Alexion Pharmaceuticals—will officially be added to the index.

But while shares of Motorola Mobility Holdings stopped trading before the market opened today, shares of Alexion won't be added to the index until Friday. That's right—for three full trading days, only 499 stocks will be trading on the S&P 500.

According to S&P indices Senior Index Analyst Howard Silverblatt, the capitalization of Motorola Mobility Holdings has been frozen as analysts re-weight the index to account for Alexion.

This is not the first time the S&P 500 will trade down a stock. "It's not that common. It happens every once in a while when a deal closes," Silverblatt told Business Insider. "You need to give [changes to the index] enough time so that you're not disrupting the market."

Then again, Silverblatt confided that a moment in which only 499 shares are actively trading for multiple days will still be a bit of an anomaly. "It's not that uncommon—then again, it's not that common."